JP5149952B2 - Optical variable filter device and filter characteristic control method thereof - Google Patents

Description

  The present invention relates to an optical variable filter device used in the fields of optical communication, optical spectroscopy, and the like, and a filter characteristic control method thereof.

  Optical variable filters are currently widely used, as represented by the optical communication field and the spectroscopic analysis field. Particularly in the field of optical communication, research and development of high transmission rates and new modulation formats are actively conducted to meet the demand for transmission capacity in recent years, and optical networks are also complicated. In such an optical network, an optical variable filter capable of changing light having a desired wavelength among optical signals is used. For example, Patent Document 1 discloses an optical variable filter that uses a two-dimensional reflective LCOS (Liquid Crystal On Silicon) liquid crystal element (hereinafter referred to as an LCOS element) as a frequency selection element.

  On the other hand, in order to meet the demand for transmission capacity in recent years, transmission rates have been increased and new modulation formats have been actively researched and developed, and optical networks have also become complicated. In such an optical network, in order to achieve optimum filtering for the transmission rate and modulation format of each optical signal, in addition to the conventional frequency selection function, the filter center frequency at the optical frequency level and the dynamics of the passband A control function is required.

US2006 / 0067611A1

  The optical variable filter device is required to have a function of changing the center frequency of the filter at the optical frequency level and changing the optical signal transmission rate and modulation format so as to obtain an optimum passband. In order to change the center frequency and the passband, light is dispersed in different directions according to the light frequency, is incident on a frequency selection element having a large number of pixels, and transmittance is controlled on and off for each pixel. Can be considered.

  However, the frequency resolution is limited to the frequency assigned to one pixel. This assigned frequency is determined by the product (D · d) of the inverse dispersion amount D, which is the frequency per unit length on the plane of the frequency selection element, and the pixel width d in the frequency dispersion direction. Therefore, to increase the resolution of the frequency selection characteristics, it is necessary to reduce the pixel width or reduce the amount of inverse dispersion, and the frequency selection element with a larger device or a larger number of pixels is required. There was a problem of becoming. Therefore, it has been desired to increase the frequency resolution without requiring an increase in the size of the device and the refinement of the frequency selection element.

  The present invention has been made in view of the problems of such a conventional optical variable filter device, and does not require an increase in the size of the device and the fineness of the frequency selection element. It is a technical problem to make it possible to change the filter characteristics.

In order to solve this problem, an optical variable filter device according to the present invention includes an incident / exit unit that emits light and emits light having a frequency selected from incident light, and the light incident on the incident / exit unit. A frequency dispersion element that spatially disperses according to frequency and combines reflected light, a light collection element that collects light dispersed by the frequency dispersion element in parallel to a two-dimensional plane, and the light collection element. A frequency selection element that is arranged at a position to receive the collected light and has a plurality of pixels arranged at least in the frequency dispersion direction, and obtains a desired frequency selection characteristic by changing the reflection characteristic of each pixel; And a frequency selection element driving unit that controls the transmission characteristics with at least four gradations or more for each frequency of incident light by driving an electrode of each pixel of the frequency selection element.

In order to solve this problem, an optical variable filter device according to the present invention includes an incident portion that receives light, a frequency dispersion element that spatially disperses light incident from the incident portion according to the frequency, and the frequency. A first condensing element for condensing the light dispersed by the dispersive element on a two-dimensional plane, and a position for receiving the light collected by the first condensing element, arranged at least in the frequency dispersion direction A frequency selection element that selects a light of an arbitrary frequency by changing a transmission characteristic of each pixel, and a frequency of an incident light by driving an electrode of each pixel of the frequency selection element A frequency selection element driving unit that performs gradation control of light transmission characteristics with gradations of at least four gradations, a second condensing element that condenses light of each frequency transmitted through the frequency selection element, and Second light collection Those comprising the frequency synthesis device for synthesizing the dispersion light condensed by a child, an emitting unit for emitting the light synthesized by the frequency synthesizer device, the.

  Here, the frequency selection element may have a pixel width in the frequency dispersion direction smaller than a beam radius of the incident light in the frequency dispersion direction.

  Here, the frequency selection element is an LCOS element having a large number of pixels arranged in at least one dimension, and the frequency selection element driver controls a voltage applied to each pixel according to frequency selection characteristics. Also good.

  Here, the frequency selection element is a liquid crystal element having a large number of pixels arranged in at least one dimension, and the frequency selection element driver controls a voltage applied to each pixel according to frequency selection characteristics. Also good.

  Here, the frequency selection element is a MEMS element having a large number of pixels arranged in at least one dimension, and the frequency selection element driver controls a voltage applied to each pixel according to frequency selection characteristics. Also good.

  In order to solve this problem, a filter characteristic control method for an optical variable filter device according to the present invention uses a pixel group including at least one pixel of the frequency selection element corresponding to each frequency for each frequency of incident light. When the ratio of the incident / outgoing light to be emitted is the transmittance of the pixel group, a desired continuous pixel group is set in a light transmissive state, and at least one first pixel adjacent to the end pixel group in the pixel group in the transmission frequency range. The bandwidth is increased by gradually increasing the transmittance of one control pixel group and at least one second control pixel group adjacent to the pixel group at the other end of the pass frequency band at the same time. .

  In order to solve this problem, a filter characteristic control method for an optical variable filter device according to the present invention uses a pixel group including at least one pixel of the frequency selection element corresponding to each frequency for each frequency of incident light. When the ratio of the incident / outgoing light to be emitted is the transmittance of the pixel group, a desired continuous pixel group is set in a light transmissive state, and at least one first control pixel at the end of the pixel group in the transmission frequency range The bandwidth is reduced by gradually decreasing the transmittance of the group and at least one second control pixel group at the other end of the pass frequency band at the same time.

  In order to solve this problem, a filter characteristic control method for an optical variable filter device according to the present invention uses a pixel group including at least one pixel of the frequency selection element corresponding to each frequency for each frequency of incident light. When the ratio of the incident / outgoing light to be emitted is the transmittance of the pixel group, the desired continuous pixel group is set in a light transmission state, and adjacent to the pixel group at the end in the direction of frequency change among the pixel group in the transmission frequency range. The light transmittance of at least one first control pixel group is gradually increased, and the transmittance of at least one second control pixel group of the pixel group at the other end of the transmission frequency range is gradually decreased. As a result, the center frequency of the transmission frequency range is changed along the frequency axis.

  According to the present invention having such characteristics, the frequency-dispersed light is made to correspond to a plurality of pixels arranged in the dispersion direction, and the transmittance of the pixels is continuously controlled in gradation. For this reason, it is not necessary to increase the size of the device and to increase the definition of the frequency selection element, and the frequency resolution can be increased. Thereby, it is possible to obtain an effect that the pass band width can be changed with high resolution and the center frequency of the pass band can be changed.

FIG. 1A is a diagram showing an optical arrangement of a reflective optical variable filter device according to the first embodiment of the present invention viewed from the x-axis direction. FIG. 1B is a diagram showing an optical arrangement from the y-axis direction of the reflective variable optical filter device according to the first embodiment of the present invention. FIG. 2A is a diagram showing an optical arrangement of the transmission type optical variable filter device according to the second embodiment of the present invention viewed from the x-axis direction. FIG. 2B is a diagram showing an optical arrangement from the y-axis direction of the transmissive variable optical filter device according to the second embodiment of the present invention. FIG. 3A is a diagram showing a two-dimensional frequency selection element used in the variable optical filter device according to the first and second embodiments of the present invention. FIG. 3B is a diagram showing a one-dimensional frequency selection element used in the variable optical filter device according to the first and second embodiments of the present invention. FIG. 4A is a diagram showing an example of a modulation method of the LCOS element used in the first embodiment of the present invention. FIG. 4B is a diagram showing another example of the modulation method of the LCOS element used in the first embodiment of the present invention. FIG. 5 is a diagram showing one pixel of the MEMS element according to the first embodiment of the present invention. FIG. 6A is a diagram showing an example of a modulation method of the LCOS element used in the second embodiment of the present invention. FIG. 6B is a diagram showing another example of the modulation method of the LCOS element used in the second embodiment of the present invention. FIG. 7A is a diagram showing a control method when the bandwidth of the bandpass filter is widened in the first and second embodiments of the present invention. FIG. 7B is a diagram showing a control method when the band width of the bandpass filter is narrowed in the first and second embodiments of the present invention. FIG. 8 is a diagram showing an example of a change in bandwidth of the optical variable filter device according to the first and second embodiments of the present invention. FIG. 9 is a diagram showing an example of the amount of change in bandwidth of the optical variable filter device according to the first and second embodiments of the present invention. FIG. 10 is a diagram showing an example of frequency resolution with respect to the number of bits for changing the transmittance of the optical variable filter device according to the first and second embodiments of the present invention. FIG. 11A is a diagram illustrating a control method when the center frequency of the bandpass filter is increased in the first and second embodiments of the present invention. FIG. 11B is a diagram showing a control method when the center frequency of the bandpass filter is lowered in the first and second embodiments of the present invention. FIG. 12A is a diagram showing an example of an increase in the center frequency of the variable optical filter device according to the first and second embodiments of the present invention. FIG. 12B is a diagram illustrating an example of a decrease in the center frequency of the optical variable filter device according to the first and second embodiments of the present invention. FIG. 13 is a diagram showing an example of an increase in the center frequency of the variable optical filter device according to the first and second embodiments of the present invention. FIG. 14 is a graph showing a state in which the bandwidth does not change. FIG. 15A is a graph showing the relationship between the change in the center frequency and the transmittance when the transmittance of the control pixel group is changed when γ = 2.7. FIG. 15B is a graph showing the relationship between the change in the center frequency and the transmittance when the transmittance of the control pixel group is changed when γ = 1.8. FIG. 15C is a graph showing the relationship between the change in the center frequency and the transmittance when the transmittance of the control pixel group is changed when γ = 1.2. FIG. 15D is a graph showing the relationship between the change in the center frequency and the transmittance when the transmittance of the control pixel group is changed when γ = 1.0. FIG. 15E is a graph showing the relationship between the change in the center frequency and the transmittance when the transmittance of the control pixel group is changed when γ = 0.5. FIG. 15F is a graph showing the relationship between the change in the center frequency and the transmittance when the transmittance of the control pixel group is changed when γ = 0.25.

(First embodiment)
1A is a side view showing the configuration of an optical element of a reflective variable optical filter device according to a first embodiment of the present invention, and FIG. 1B is a side view seen from the y-axis direction. . The incident light is, for example, WDM signal light in which optical signals having frequencies f _{1 to} f _n are multiplexed. The WDM light is emitted from the collimating lens 12 through the optical fiber 11. The light incident on the collimating lens 13 is emitted to the outside from the optical fiber 14. Light emitted from the collimating lens 12 is parallel to the z-axis direction and is incident on the frequency dispersion element 15. The frequency dispersion element 15 disperses light in different directions on the xz plane according to the frequency. Here, the frequency dispersion element 15 may be a diffraction grating, or a prism or the like. Moreover, the structure which combined the diffraction grating and the prism may be sufficient. The light dispersed by the frequency dispersion element 15 in this manner is given to the lens 16 that is a light condensing element. The lens 16 is a condensing element that condenses the light dispersed on the xz plane in parallel with the z-axis direction, and the condensed light is incident on the frequency selection element 17 perpendicularly.

In FIG. 1B, the light having the lowest frequency f ₁ and the highest frequency f _n is illustrated here. However, since the incident light is WDM signal light having a large number of spectrums between frequencies f _{1 to} f _n , xz The WDM signal light developed along the plane is applied to the frequency selection element 17 in a band shape. The frequency selection element 17 selectively reflects incident light, and the selection characteristics of the optical filter are determined in accordance with the reflection characteristics. Details will be described later. The light reflected by the frequency selection element 17 is added to the lens 16 through the same path, and is added to the frequency dispersion element 15 again. The frequency dispersion element 15 combines the reflected light and focuses it in the same direction as the original incident light, and emits the focused light from the optical fiber 14 via the collimator lens 13. Here, the optical fibers 11 and 14 and the collimating lenses 12 and 13 constitute an incident / exit section that receives light and emits the selected light.

  In this embodiment, the optical axes of the incident light and the outgoing light are separated, but the optical axes are made common, the incoming / outgoing light is guided to the same fiber, and the incoming / outgoing light is separated by a circulator. 14 may be used.

(Second Embodiment)
Next, a transmissive variable optical filter device according to a second embodiment of the present invention will be described. 2A is a side view showing the configuration of the optical element of the transmissive variable optical filter device according to the first embodiment of the present invention, and FIG. 2B is a side view seen from the y-axis direction. . Also in FIG. 2A, the incident light is the WDM signal described in the first embodiment, is incident on the collimator lens 22 from the optical fiber 21, and is given to the first frequency dispersion element 23 as a parallel light beam. The optical fiber 21 and the collimating lens 22 constitute an incident part for receiving WDM signal light. Similar to the frequency dispersion element 15, the frequency dispersion element 23 can be realized by a diffraction grating, a prism, or a combination of a diffraction grating and a prism. As shown in FIG. 2B, the frequency dispersion element 23 emits light in different directions on the xz plane according to the frequency of light. All of this light is incident on the lens 24. The lens 24 is a first condensing element that condenses the light dispersed on the xz plane in parallel with the z-axis direction. A frequency selection element 25 is disposed perpendicular to the optical axis of the lens 24. The frequency selection element 25 partially transmits incident light, and details will be described later. The light transmitted through the frequency selection element 25 is incident on the lens 26. The lens 24, the frequency dispersion element 23, the lens 26, and the frequency synthesis element 27 are plane symmetric with respect to the xy plane at the center of the frequency selection element 25. The lens 26 is a second condensing element that condenses parallel light on the xz plane, and the frequency synthesizing element 27 synthesizes and emits light of frequency components from different directions. The light synthesized by the frequency synthesizing element 27 is given to the optical fiber 29 via the collimating lens 28. The collimating lens 28 and the optical fiber 29 constitute an emission part that emits light of a selected frequency.

(Configuration of frequency selection element)
Next, frequency selection elements 17 and 25 used in the optical variable filter device according to the first and second embodiments will be described. In the first and second embodiments, when incident light is dispersed on the xz plane according to the frequency and is incident on the frequency selection elements 17 and 25 as band-shaped light, the incident area is rectangular as shown in FIG. 3A. Region R. In the optical variable filter device according to the first embodiment, light having an arbitrary frequency can be selected by selecting pixels to be reflected. In the optical variable filter device according to the second embodiment, light having an arbitrary frequency can be selected by selecting a pixel to be transmitted. A setting unit 30 is connected to the frequency selection elements 17 and 25 via a driver 31. The setting unit 30 determines a pixel that reflects or transmits light on the xy plane according to a selected frequency. The driver 31 includes a D / A converter that converts an input digital signal into a voltage to be applied to the pixel. The setting unit 30 and the driver 31 constitute a frequency selection element driving unit that controls the characteristics of pixels at predetermined positions in the x-axis and y-axis directions by driving the electrodes of the pixels arranged in the xy direction of the frequency selection element. doing.

  Next, a specific example of the frequency selection element 17 will be described. A first example of the frequency selection element 17 is a two-dimensional reflective LCOS (Liquid Crystal On Silicon) liquid crystal element (hereinafter referred to as an LCOS element) 17A1. Since the two-dimensional LCOS element 17A1 incorporates a liquid crystal modulation driver on the back surface of each pixel, the number of pixels can be increased, and for example, the two-dimensional LCOS element 17A1 can be composed of a large number of 1000 × 1000 grid pixels. In the LCOS element 17A1, a light beam is incident on a different position for each frequency. Therefore, if the pixel at that position is in a reflective state, light of that frequency can be selected.

  Here, a phase modulation method which is one of the modulation methods in the LCOS element 17A1 will be described. FIG. 4A is a schematic diagram showing the LCOS element 17A1, and is configured by laminating a transparent electrode 41, a liquid crystal 42, and a back reflecting electrode 43 along the z-axis from the light incident surface. As shown in FIG. 3A, since the LCOS element 17A1 corresponds to a plurality of pixels in the y-axis direction at a position corresponding to one frequency, between the transparent electrode 41 and the back reflective electrode 43 for these pixels. By applying a voltage, unevenness of the refractive index can be formed, and a diffraction phenomenon can be expressed. The diffraction angle of each frequency component is controlled independently, and the input light of a specific frequency is reflected in the incident direction as it is, or the light of other frequency components is diffracted as unnecessary light, in a direction different from the incident direction. It can reflect light. Then, by controlling the voltage applied to each pixel in an analog manner, it is possible to continuously control the gradation of the required pixels from the reflective state to the non-reflective state. If an arbitrary pixel is in a reflection state in the x-axis direction, light having an arbitrary frequency of incident light can be selected. Here, the reflectance control for each frequency component is not simply on / off control, but is controlled to at least four gradations or more.

Next, an intensity modulation method which is another modulation method of the LCOS element 17A1 will be described. FIG. 4B is a diagram illustrating a frequency selection method using an intensity modulation method, and a polarizer 44 is disposed on a surface on which incident light is incident. The polarizer 44 changes the incident light into a specific polarization state indicated by ◯ in the drawing and enters the reflective LCOS element 17A1. Also in this case, the LCOS element 17A1 includes the transparent electrode 41, the liquid crystal 42, and the back reflective electrode 43. When light is incident on the LCOS element 17A1, it is possible to control the birefringence difference of the liquid crystal between the electrodes depending on the voltage application state. In the intensity modulation method, the same voltage is simultaneously applied to pixels arranged in the y-axis direction in FIG. 3A, and the polarization state of the reflected light can be varied by controlling the polarization state of the applied pixels. Here, it is determined whether the polarization plane is rotated or held when the voltage is controlled by the alignment component of the liquid crystal molecules. For example, if the plane of polarization is maintained when no voltage is applied, the light indicated by the ◯ state in the figure is reflected as it is. On the other hand, when a voltage is applied, the polarization plane rotates and reflects, so that the reflected light is shielded by the polarizer 44 . Therefore, the voltage applied to the pixel can be controlled in an analog manner, and the reflectance can be continuously controlled in gradation from the reflected state to the non-reflected state. If an arbitrary pixel is in a reflection state in the x-axis direction, light having an arbitrary frequency of incident light can be selected. Also in this case, the control of the reflectance for each frequency component is not a simple on / off control, but at least four gradations or more.

  As a second example of the frequency selection element 17, a liquid crystal element 17A2 having a reflective two-dimensional electrode array not having an LCOS structure will be described. In the case of the LCOS element, a liquid crystal driver is built in the back of the pixel, but the two-dimensional electrode array liquid crystal element 17A2 is equipped with a liquid crystal modulation driver outside the element. Other configurations are the same as those of the LCOS element, and the above-described phase modulation method and intensity modulation method can be realized. Further, the gradation can be continuously controlled by changing the voltage level of the pixel in an analog manner.

  As a third example of the frequency selection element 17, a two-dimensional MEMS element 17A3 will be described. A MEMS element in which a number of MEMS mirrors are two-dimensionally arranged has been put into practical use as a digital micro element (DMD). All the pixels in one column in the y-axis direction of the MEMS mirror are assumed to correspond to a certain optical frequency of the WDM signal. Even when MEMS is used, pixels of a plurality of MEMS elements are associated with one frequency band. Therefore, the voltage applied to a large number of pixels corresponding to one frequency is controlled and phase-modulated to reflect the reflection. The rate can be different. Further, as shown in FIG. 5, intensity modulation is performed by rotating each pixel of the MEMS element around the x-axis or y-axis, and the angle of the mirror at the voltage level applied to each pixel can be controlled. The amount of light can be set arbitrarily. Therefore, also in this case, gradation control of the light intensity level in the selected frequency band can be performed.

  Next, as a fourth example of the frequency selection element 17, a one-dimensional LCOS element 17B1 will be described. The frequency selection element 17B1 is an LCOS element in which a number of elongated pixels are arranged in the x-axis direction as shown in FIG. 3B, and WDM light dispersed according to the frequency is incident along the x-axis. Also in this case, the setting unit 32 and the driver 33 drive the electrode of each pixel arranged in the x-axis direction of the frequency selection element 17 to control the characteristics of the pixel at a predetermined position in the x-axis direction. Is configured. The LCOS element 17B1 is driven by the setting unit 32 via the driver 33. In this case, the phase modulation method described above cannot be used, and the frequency is selected only by the intensity modulation method.

  As a fifth example of the frequency selection element 17, a liquid crystal element 17B2 having a reflective one-dimensional electrode array can be used. Also in this case, the phase modulation method cannot be used, and the frequency is selected only by the intensity modulation method.

  Further, as a sixth example of the frequency selection element 17, a reflection type one-dimensional MEMS mirror element 17B3 can be used. Also in this case, the intensity phase modulation method cannot be used, and the frequency is selected only by the intensity modulation method.

  Next, the transmission-type frequency selection element 25 used in the wavelength tunable filter device according to the second embodiment will be described. As a first example of the frequency selection element 25, a transmission type two-dimensional LCOS element 25A1 can be used. Also in the LCOS element 25A1, since the light beam is incident on a different position for each frequency, the light signal can be selected by setting the pixel at that position to the transmission state.

Here, a phase modulation method which is one of the modulation methods in the LCOS element 25A1 will be described. FIG. 6A is a schematic diagram showing an LCOS element, which is configured by laminating a transparent electrode 51, a liquid crystal 52, and a transparent electrode 53 along the z-axis from a light incident surface. Since the LCOS element 25A1 corresponds to a plurality of pixels in the y-axis direction at a position corresponding to one frequency, the refractive index is applied by applying a voltage between the transparent electrode 51 and the transparent electrode 53 for the plurality of pixels. As a result, the diffraction phenomenon can be expressed. Then, the diffraction angle of each frequency component is controlled independently, and the input light of a specific frequency goes straight in the z-axis direction and is transmitted as it is, or the light of other frequency components is diffracted as unnecessary light. Light can be diffracted in different directions. For this reason, by controlling the voltage applied to each pixel, it is possible to make a necessary pixel in a transmissive state without diffracting it. Here, the transmittance control for each frequency component is not simply on / off control, but is controlled to at least four gradations or more.

Next, an intensity modulation method which is another modulation method of the LCOS element will be described. FIG. 6B is a diagram illustrating a wavelength selection method using an intensity modulation method, and a polarizer 54 is disposed on a surface on which incident light is incident. The polarizer 54 makes incident light enter the LCOS element 25A1 in a specific polarization state indicated by ◯ in the drawing. Also in this case, the LCOS element is constituted by the transparent electrode 51, the liquid crystal 52 and the transparent electrode 53. A polarizer 55 is disposed on the optical axis of the outgoing light that has passed through the LCOS element. The polarizer 55 emits only light in a specific polarization state indicated by a circle in the figure. When light is incident on the LCOS element, the difference in birefringence of the liquid crystal between the electrodes can be controlled by applying a voltage. Therefore, the polarization state of transmitted light can be varied by independently controlling the polarization state of the applied voltage. Here, it is determined whether the polarization plane is rotated or held when the voltage is controlled by the alignment component of the liquid crystal molecules. For example, if the plane of polarization is maintained when no voltage is applied, the light indicated by the ◯ state in the figure is transmitted as it is. On the other hand, when a voltage is applied, the polarization plane rotates and transmits, so that transmitted light is shielded by the polarizer 55. Therefore, the incident light can be selected by controlling the voltage applied to the pixel. Here, if an arbitrary pixel is in a transmissive state, light having a frequency corresponding to that pixel can be selected. In this case as well, the transmittance control for each frequency component is not simple on / off control, but at least four gradations or more.

As a second example of the frequency selection element 25, a liquid crystal element 25A2 having a transmissive two-dimensional electrode array that does not have an LCOS structure can be used. In the case of the LCOS element, a liquid crystal driver is built in the back surface of the pixel, but the two-dimensional electrode array liquid crystal element 25A2 is provided with a driver for liquid crystal modulation outside the element. Other configurations are the same as those of the LCOS element, and the above-described phase modulation method and intensity modulation method can be realized. Further, the transmittance can be continuously controlled by changing the voltage level of the pixel in an analog manner.

  Next, a one-dimensional LCOS element 25B1 will be described as a third example of the frequency selection element 25. FIG. As shown in FIG. 3B, the frequency selection element 25B1 is a transmissive LCOS element in which a number of elongated pixels are arranged in the x-axis direction, and WDM light dispersed according to the frequency is incident along the x-axis. Also in this case, the setting unit 32 and the driver 33 provide a frequency selection element driving unit that controls the characteristics of the pixels at predetermined positions in the x-axis direction by driving the electrodes of the pixels arranged in the x-axis direction of the frequency selection elements. The LCOS element 25B1 is driven from the setting unit 32 via the driver 33. In this case, the phase modulation method described above cannot be used, and the frequency is selected only by the intensity modulation method.

  As a fourth example of the frequency selection element 25, an electrode array liquid crystal element 25B2 having a transmission type one-dimensional electrode array can be used. Also in this case, the phase modulation method cannot be used, and the frequency is selected only by the intensity modulation method.

(Bandwidth change)
Next, details of frequency control of the optical variable filter device according to the first and second embodiments of the present invention will be described. In the following description, since a part of incident light is reflected by each pixel of the frequency selection element and returns to the output side even in the reflection type optical variable filter device as in the first embodiment. This will be described as the transmittance. Further, in the following description, even if it is a two-dimensional frequency selection element, all the pixels corresponding to the same frequency, that is, the pixels along all the y axes having the same x coordinate are considered as one pixel group. That is, the frequency selection element will be described assuming that the pixel groups P _{1 to} P _m are arranged in the x-axis direction. Note that one pixel corresponds to this pixel group in the one-dimensional frequency selection element. Each of the pixel groups P _{1 to} P _m has an individual transmittance.

In the first state, the transmittance of the pixel groups P _{i + 1 to} P _{i + k} (1 <= i, k <= m) is set to 1, and the transmittances of the other pixel groups are set to 0. If it carries out like this, it will become a band pass filter which passes the frequency band of the transmittance | permeability. Here, when the bandwidth is enlarged, the transmittance of the pixel group adjacent to both ends thereof is increased simultaneously and continuously. Thus it means a pixel group for changing the transmittance and the control group of pixels, and _{P i, P i + k +} 1 , respectively first, second control group of pixels. FIG. 7 is a graph in which the horizontal axis represents the frequency and the vertical axis represents the transmittance of the pixel group of the frequency selection element. As shown in this graph, the transmittances of the control pixel groups P _i and P _{i + k + 1} are continuously changed in an analog manner. For example, as shown in FIG. 8, when the transmittances of the control pixel groups P _i and P _{i + k + 1} are simultaneously increased from 0 to 1 by 0.25 units as A to E, the transmittance is 0. If the transmittance is continuously increased to ˜1 at the same time, the bandwidth can be increased accordingly. FIG. 8 is an example in the case where the following condition is satisfied in the optical filter used for the WDM signal having the channel width of 50 GHz.
(1) The amount of linear dispersion on the plane of the frequency selection element, that is, the width on the pixel per 1 GHz is 2.89 μm / GHz,
(2) The light beam radius w is 22.6 μm,
(3) The pixel width in the frequency dispersion direction on the frequency selection element is set to 8.5 μm,
(4) One control pixel group is provided on each of the high frequency side and the low frequency side.
In this example, the transmission characteristics change from the bandwidth of the center frequency ± 25 GHz to ± 28 GHz as shown by the curves A to E. Thus, if the transmittance of the pixel groups at both ends is continuously increased, the bandwidth can be continuously increased.

  FIG. 9 shows the transmittance of the control pixel group, the bandwidth of −3 dB, and the bandwidth of −0.5 dB. It can be seen that the bandwidth is increased in proportion to the transmittance of the control pixel group. .

Next, from the initial state described above, out of the pixel groups P _{i + 1 to} P _{i + k} constituting the band pass filter, a pair of pixel groups P _{i + 1} , P _{i + k} (first and second) If the transmittance of the control pixel group) is gradually decreased from 1 to 0 as shown in FIG. 7B, the bandwidth can be reduced.

  In this way, by continuously changing the transmittance of a pair of pixels on the high frequency side and the low frequency side of a predetermined bandwidth in the same direction, the bandwidth of the band pass filter can be widened or reduced. Although the number of control pixel groups on both sides is one here, the present invention is not limited to this, and the slope characteristics of the filter of the bandpass filter can be changed by simultaneously controlling a plurality of pixel groups. In addition, when the variable width of the passband is set to be equal to or higher than the frequency corresponding to the two-pixel group, the variable width of the passband can be controlled continuously without limitation by sequentially shifting the control pixel group to adjacent pixels. It becomes possible.

  Here, the frequency resolution of the frequency selection element is determined by the voltage applied to the element. In the intensity modulation method, the voltage gradation, that is, the number of bits of the D / A converter included in the driver 31 or 33 directly corresponds to the resolution. FIG. 10 shows a resolution that can be set with respect to the number of input bits in the case of intensity modulation. Also in this case, the conditions are (1) to (4) described above. As can be seen from this figure, frequency control in MHz is possible by setting the number of bits to 13 or more.

(Center frequency shift)
Next, a control method for shifting the center frequency of the bandpass filter will be described. First, the transmittance of the pixel groups P _{i + 1 to} P _{i + k} is 1 and the others are all 0, and a band pass filter in the frequency range of light incident on the pixel groups P _{i + 1 to} P _{i + k} is constructed. It shall be. Here, if shifting the passband to the high frequency side, the group of pixels P _{i + k + 1} adjacent to the highest frequency group of pixels P _{i + k} a, as shown in FIG. 11 (first control group of pixels) While gradually increasing the transmittance, the transmittance of the pixel group P _{i + 1} (second control pixel group) having the lowest frequency is decreased. At this time, it is continuously changed so that the total transmittance of the control pixel groups P _{i + 1} and P _{i + k + 1} becomes 1. In this way, assuming that the curves when the transmittance of the control pixel group P _{i + 1} decreases from 1 to 0 in units of 0.25 are A to E, the center frequency of the bandpass filter as shown in FIGS. Can be continuously increased. At this time, if the total transmittance change amount of the control pixel group is set to 0, the bandwidth does not change as shown in FIG.

Next, a control method for reducing the center frequency of the filter from the above-described initial state will be described. In this case, the transmittance of the pixel group P _i adjacent to the pixel group P _{i + 1} (first control pixel group) on the low frequency side in the changing direction is increased, and the pixel group P _{i + k} on the high frequency side is increased. Control is continuously performed so as to reduce the transmittance of the (second control pixel group). Here, FIG. 11B shows a state where the total of the transmittances of the two control pixel groups P _i and P _{i + k} is changed while maintaining the condition that it is 1. In this way, assuming that the curves when the transmittance of the control pixel group P _i increases from 0 to 1 in 0.25 units are A to E, as shown in FIG. 12B, it corresponds to the change in the transmittance of the control pixel group. Although the center frequency is lowered, the frequency can be shifted to the lower frequency side while keeping the filter bandwidth constant.

  In any of the above cases, when the variable width of the center frequency exceeds the frequency per pixel, the variable width of the center frequency is controlled continuously without limitation by sequentially shifting the control pixel group to the adjacent pixel group. be able to. Further, it is possible to control in the MHz order by increasing the number of bits of the voltage gradation applied to the frequency selection element.

Next, optical design conditions in which this control method is effective will be described. In this embodiment, since the intermediate value of the transmittance of each pixel group is reflected in the filter shape, such a continuous characteristic change is caused by the width in the frequency dispersion direction and the input beam diameter of each pixel of the control pixel group. Possible conditions are determined. That is, if the width of the control pixel group is larger than the input beam diameter of each frequency component of the WDM signal light, it is assumed that the transmittance designed for the control pixel group is reflected in the filter waveform as it is and distortion occurs. Here, the width of the pixel in the frequency dispersion direction in the frequency selection element is d, and the range of the light intensity up to 1 / e ^{2 of the} peak is the beam radius w of each frequency component. At this time, a parameter γ defined by the pixel width d and the beam radius w is introduced.
γ = w / d

15A to 15F show the case where the transmittance of the control pixel group on the low frequency side is changed from 0 to 0.75 in units of 0.25 in order to cope with the bandwidth under the above-described conditions or to lower the center frequency. The relationship between the transmission frequency and the transmittance is shown. 15A is γ = 2.7, FIG. 15B is γ = 1.8, FIG. 15C is γ = 1.2, FIG. 15D is γ = 1.0, FIG. 15E is γ = 0.5, and FIG. This is the case when γ = 0.25. As can be seen from these figures, waveform distortion does not occur when γ is 1.0 or more, but when γ is less than 1, that is, when γ is 0.5 or 0.25, the transmittance response is the filter waveform. This is reflected in the distortion. Therefore, γ is preferably 1 or more, that is, the pixel width d is smaller than the input beam radius w.

  In the first and second embodiments described above, the incident light is WDM signal light, but the incident light is not limited to WDM signal light. That is, the present invention can be used in various applications for filtering arbitrary light, for example, in the fields of tunable laser and optical spectroscopic analysis.

  As described above in detail, according to the present invention, the frequency selection characteristic of incident light can be changed by changing the reflection characteristic and transmission characteristic of the frequency selection element for each pixel. Thereby, light can be used as a main component of a node having an add / drop function of WDM light or a component of an optical spectroscopic device.

11, 14, 21, 29 Optical fiber 12, 13, 22, 28 Collimating lens 16, 24, 26 Lens 15, 23 Frequency dispersion element 17, 25 Frequency selection element 17A1, 25A1 Two-dimensional LCOS element 17A1, 25A2 Two-dimensional electrode array Liquid crystal element 17A3 Two-dimensional MEMS element 17B1, 25B1 One-dimensional LCOS element 17B1, 25B2 One-dimensional electrode array Liquid crystal element 17B3 One-dimensional MEMS element 27 Frequency synthesis element 30, 32 Setting unit 31, 33 Driver 41, 51, 53 Transparent electrode 42, 52 Liquid crystal 43 Reflective electrode 44, 54, 55 Polarizer

Claims (9)

An incident / exit section that emits light and emits light of a selected frequency among incident light; and
A frequency dispersion element that spatially disperses light incident on the incident / exit section according to its frequency, and combines reflected light;
A condensing element that condenses the light dispersed by the frequency dispersive element in parallel to a two-dimensional plane;
It has a large number of pixels arranged at a position for receiving the light condensed by the light condensing element and arranged at least in the frequency dispersion direction, and obtains a desired frequency selection characteristic by changing the reflection characteristic of each pixel. A frequency selective element;
A variable optical filter device comprising: a frequency selection element driving unit that controls a transmission characteristic at a gradation of at least 4 gradations for each frequency of incident light by driving an electrode of each pixel of the frequency selection element . An incident part for incident light;
A frequency dispersion element that spatially disperses light incident from the incident portion according to the frequency;
A first condensing element that condenses the light dispersed by the frequency dispersive element on a two-dimensional plane;
It has a large number of pixels arranged at a position for receiving the light collected by the first light condensing element and arranged at least in the frequency dispersion direction. By changing the transmission characteristics of each pixel, an arbitrary frequency can be obtained. A frequency selection element for selecting light;
A frequency selection element driving unit that controls a light transmission characteristic at a gradation of at least four gradations or more for each frequency of incident light by driving an electrode of each pixel of the frequency selection element;
A second condensing element that condenses light of each frequency transmitted through the frequency selection element;
A frequency synthesizing element that synthesizes the dispersed light collected by the second condensing element;
An optical variable filter device comprising: an emission unit that emits light synthesized by the frequency synthesis element.   The variable optical filter device according to claim 1, wherein the frequency selection element has a pixel width in a frequency dispersion direction smaller than a beam radius of the incident light in the frequency dispersion direction. The frequency selection element is an LCOS element having a large number of pixels arranged in at least one dimension,
The variable optical filter device according to claim 1, wherein the frequency selection element driving unit controls a voltage applied to each pixel according to a frequency selection characteristic. The frequency selection element is a liquid crystal element having a plurality of pixels arranged in at least one dimension,
The variable optical filter device according to claim 1, wherein the frequency selection element driving unit controls a voltage applied to each pixel according to a frequency selection characteristic. The frequency selective element is a MEMS element having a plurality of pixels arranged in at least one dimension,
The optical variable filter device according to claim 1, wherein the frequency selection element driving unit controls a voltage applied to each pixel according to a frequency selection characteristic. A filter characteristic control method in the optical variable filter device according to claim 1 or 2,
For each frequency of incident light, when the ratio of incident / exit light emitted through a pixel group composed of at least one pixel of the frequency selection element corresponding to each frequency is the transmittance of the pixel group, a desired continuous The pixel group is in a light transmission state,
At least one first control pixel group adjacent to the pixel group at the end of the pixel group in the transmission frequency range and at least one second control pixel adjacent to the pixel group at the other end of the pass frequency band A filter characteristic control method for an optical variable filter device that expands a bandwidth by gradually increasing the transmittance of a group simultaneously. A filter characteristic control method in the optical variable filter device according to claim 1 or 2,
For each frequency of incident light, when the ratio of incident / exit light emitted through a pixel group composed of at least one pixel of the frequency selection element corresponding to each frequency is the transmittance of the pixel group, a desired continuous The pixel group is in a light transmission state,
The transmittance of at least one first control pixel group at the end of the pixel group in the transmission frequency range and at least one second control pixel group at the other end of the pass frequency band is gradually decreased simultaneously. The filter characteristic control method of the optical variable filter apparatus which reduces a bandwidth by making it perform. A filter characteristic control method in the optical variable filter device according to claim 1 or 2,
For each frequency of incident light, when the ratio of incident / exit light emitted through a pixel group composed of at least one pixel of the frequency selection element corresponding to each frequency is the transmittance of the pixel group, a desired continuous The pixel group is in a light transmission state,
Gradually increasing the light transmittance of at least one first control pixel group adjacent to the pixel group at the end in the direction of frequency change among the pixel group in the transmission frequency range;
An optical variable filter that changes the center frequency of the transmission frequency range along the frequency axis by gradually decreasing the transmittance of at least one second control pixel group of the pixel group at the other end of the transmission frequency range. Device filter characteristic control method.

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